1. Field of the Invention
This invention relates to positioning systems. More particularly, this invention relates to a solution for obtaining a first location of a receiver in a satellite positioning system, for instance when the receiver is switched on.
2. Description of the Prior Art
The GPS-system (Global Positioning system) is an example of a prior art positioning system, where the position of a receiver is calculated based on signals transmitted from satellites to the receiver. In the following the invention will, by way of example, be explained mainly in connection with the GPS-system. It should, however, be observed that the present invention can also be utilized in other positioning systems.
One problem with prior art positioning systems is the operations needed in order to obtain the first location of the receiver when the receiver performs a so-called warm-start. A warm-start refers to a situation where the receiver has in its memory information indicating the orbits of the satellites (though this information might be old), but the receiver does not have any precise information of its location when it starts the process of finding out its position. Such a situation may occur for instance if the receiver has been moved from one location to another while being switched off. Another alternative is that the receiver has not been able to receive signals transmitted by the satellites while it has been moved from one location to another (for instance inside an airplane). In such a situation the receiver is not aware of the satellites available at the location of the receiver when the warm-start is initiated. Instead, the receiver has to first search through all the available frequencies and PRN code phases in order to identify the available satellites before it can start the calculations needed to determine its location based on signals received from the satellites. The search for available satellites is rather slow and it also consumes energy, which is a drawback especially for portable receivers whose energy source consists of a battery. In practice it is usually necessary to search for only one satellite at a time in order to reduce the energy consumption. Thus a search for all 32 possible satellites and PRN codes within the GPS-system will require a time period of several tens of seconds.
An object of the present invention is to solve the above mentioned drawback and to provide a solution which makes it possible to minimize the operations needed in order to determine the location of a receiver when the receiver performs a warm-start.
A further object of the present invention is to provide a solution which makes it possible to minimize the energy consumption and time needed in order to determine the location of a receiver when the receiver performs a warm-start.
The objects of the invention are achieved with a method for determining positional coordinates of a receiver receiving signals from satellites in a satellite navigation system, said method comprising: carrying out measurements in order to obtain a measurement result describing properties of the magnetic field of the earth at the location of the receiver, obtaining an estimated latitude based on said measurement result, comparing the local time indicated by a clock in said receiver and the satellite time in order to identify the time zone where said receiver is located, obtaining an estimated longitude based on the identified time zone, identifying a group of available satellites based on the estimated latitude, estimated longitude and information indicating orbits of the satellites within said positioning system, performing a search for signals transmitted by satellites which belong to said group of available satellites, and determining the positional coordinates of the receiver based on signals received from satellites which belong to said group of satellites.
According to a second aspect, the objects of the present invention are achieved with a receiver of a satellite positioning system comprising: reception means for receiving signals transmitted by satellites, means for measuring the magnetic field of the earth at the location of said receiver in order to obtain a measurement result describing properties of said magnetic field and for obtaining an estimated latitude based on the measurement result, a clock for keeping track of the local time at the location of the receiver, and means for comparing the local time indicated by said clock and the satellite time in order to identify a time zone where said receiver is located and for obtaining an estimated longitude based on said identified time zone, said receiver being responsive to said estimated latitude, estimated longitude and information indicating orbits of the satellites for identifying a group of available satellites, for performing a search for signals transmitted by satellites which belong to said group of satellites, and for calculating the positional coordinates of the receiver based on signals received from satellites which belong to said group of satellites.
The present invention utilizes the magnetic field of the earth and the difference between the local time fed into the receiver by the user and the satellite time in order to determine a first estimated position of a receiver. Such an estimated position is not very accurate. However, the estimated position obtained in such a way is accurate enough in order to identify the satellites available at the location of the receiver. This can be done as the receiver in a warm-start situation has stored information about the orbits of the satellites in its memory, which means that the available satellites can be identified as soon as an estimated position has been calculated.
The present invention is very useful in order to determine the first location of a receiver, in other words in a situation where no exact position is available, as the search of available satellites can be reduced significantly. It is sufficient to establish an estimated position, to use orbit information in order to identify available satellites and to restrict the search only to the available satellites. Thus significant savings in time and energy consumption can be achieved.